The guard fence is blocked out from the posts with routed timber or composite blockouts (nominally 6-in x 8-in x 14-in). These blockouts reduce vehicle snagging on the posts and the likelihood of a vehicle vaulting over the barrier by maintaining the rail height during the initial stages of post deflection.

It is acceptable to use double blockouts (up to 16-in in depth) to increase the post offset to avoid obstacles such as utilities. There is no limit to the number of posts that can have double blockouts installed, except terminals, unless approved by the manufacturer. Under special circumstances, such as avoiding buried obstacles that are not relocated, it is also acceptable to install triple blockouts (up to 24- in offset) for one post for every 75-ft of guard fence.